This invention relates to down-hole bent motor housings for use in drilling a borehole along a curve.
It is well known, in the field of directional drilling of boreholes, to drill a borehole along a curve using a mud motor having a bent housing for angularly offsetting the rotational axis of the drill bit relative to the longitudinal axis of the drill string so as to cause the drill bit to engage the wall of the borehole on one side and to thereby effect drilling along a curve whose curvature is determined by the angular offset of the bend in the housing. However, the angle of the bend in the housing is generally limited to about 11/2.degree. by the articulated motor shaft which extends through the housing to the drill bit.
Furthermore modern drilling methods can impose severe directional drilling demands which cannot be n1et by conventional bent housing mud motors. For example, in drilling a curved section of borehole in order to deflect the direction of the borehole from vertical (that is 0.degree. inclination) to horizontal (that is 90.degree. inclination), it may be necessary to increase the inclination of the borehole by, say, 12.degree. for every hundred feet drilled. This degree of curvature cannot be attained using a conventional bent housing mud motor.
It is an object of the invention to provide a down-hole bent motor housing permitting directional drilling at an increased curvature.
According to the present invention, there is provided a down-hole bent motor housing for use in drilling a borehole along a curve, the housing being elongate and having a top end by which it is to be connected to the end of a drill string within the borehole, a bottom end on which a drill bit is to be mounted for drilling the borehole, a bend intermediate the top and bottom ends for angularly offsetting the rotational axis of the drill bit relative to the longitudinal axis of the drill string to cause the drill bit to engage the wall of the borehole on one side, and a longitudinal bore extending from the top end to the bottom end of the housing and provided for passage of a motor shaft, wherein the housing has a stand-off portion in the vicinity of the bend for bearing against the wall of the borehole on the opposite side to that engaged by the drill bit so as to cause the angle of tilt of the rotational axis of the drill bit relative to the axis of the borehole to exceed the angle of the bend in the housing.
The provision of the stand-off portion enables the angle of tilt of the rotational axis of the drill bit relative to the axis of the borehole to be increased relative to the angle of tilt obtainable using a conventional down-hole bent motor housing having a bend of the same angular offset. This increases the interference between the drill bit and the side of the borehole, as well as increasing the side force exerted by the drill bit on the formation being drilled, thus enabling a greater curvature to be obtained during drilling.
In a preferred embodiment of the invention the bore comprises a first rectilinear passage extending through a top portion of the housing and a second rectilinear passage extending through a bottom portion of the housing, the two passages meeting at the bend at an angle corresponding to the angular offset of the bend, and the provision of the stand-off portion bearing against the wall of the borehole resulting in the first passage being slightly inclined relative to the axis of the borehole in use.
Furthermore it is preferred that the housing comprises a top cylindrical housing part and a bottom cylindrical housing part, the two parts meeting at the bend at an angle corresponding to the angular offset of the bend, and the first and second passages extending respectively through the top and bottom housing parts and being coaxial therewith at least in the vicinity of the top and bottom ends of the housing.
Advantageously each of the top and bottom housing parts comprises a concentric housing section remote from the bend in which the associated passage is coaxially disposed and an eccentric housing section adjacent the bend in which the passage is eccentrically disposed so as to provide a thickened region of the housing wall, constituting said stand-off portion, on the side on which the housing bears against the wall of the borehole.
The eccentricity of the passage in the eccentric housing section of each of the top and bottom housing parts is preferably such that the thickness of the housing wall is at a maximum on the side on which the housing bears against the wall of the borehole and at a minimum on the diametrically opposite side, whereas the concentric housing section of each of the top and bottom housing parts has a wall thickness which is less than the minimum wall thickness of the associated eccentric housing section. Such an arrangement provides enhanced stiffness at the bend.
It is also preferred that the housing is formed from a single piece of metal by machining. This also increases structural rigidity.
In a development of the invention the bottom housing part is rotatable with respect to the top housing part to vary the angular offset, for example by means of an arrangement as described in British Patent No. 1494273.
The invention also provides a down-hole mud motor incorporating such a motor housing.